Definition of "mysteriousness"

This may seem strange; and indeed it is so: and it is one of the great mysteriousnesses of the Gospel.

1651, Jeremy Taylor, Twenty-Seven Sermons for the Summer Half-Year, Sermon II, Part II in ΕΝΙΑΥΤΟΣ: A Course of Sermons for All the Sundays of the Year, London: Tyler & Royston, 1668, p. 14

This sort of mysteriousness, which is always so becoming in a hero, threw a fresh grace in Catherine's imagination around his person and manners, and increased her anxiety to know more of him.

1803 (date written), [Jane Austen], chapter V, in Northanger Abbey; published in Northanger Abbey: And Persuasion. […], volumes (please specify |volume=I or II), London: John Murray, […], 20 December 1817 (indicated as 1818)

His children had urged him to travel: Mary Chivers had felt sure it would do him good to go abroad and "see the galleries." The very mysteriousness of such a cure made her the more confident of its efficacy.

1920, Edith Wharton, chapter XXXIV, in The Age of Innocence, New York, N.Y., London: D[aniel] Appleton and Company, book II, page 354

And all this is not changed by adding "mysterious" experiences to "manifest" ones, self-confident in the wisdom that recognizes a secret compartment in things, reserved for the initiated, and holds the key, O mysteriousness without mystery, O piling up of information! It, it, it!

1970, Martin Buber, translated by Walter Kaufmann, I and Thou, New York: Charles Scribner's Sons, First Part, p. 56